It was the type of conversation that would make necks crane in a movie queue.

"But how do i tell him that I don't want to do it when I don't?" the young woman asked nervously.

"If you have an honest relationship you just tell him," came the quick response.

"But what if we don't? How can I face him the next day if we don't sleep together? I still want him to like me."

A scene from "Teen Confessions?" No way.

It was a scene from a course on human sexuality that has been playing to packed classrooms in Falls Church for nearly a decade. And unlike other Northern Virginia school districts, where sex education is nearly nonexistent and where proposals for such courses immediately draw vocal and usually successful opposition, the Falls Church program not only has survived it has thrived.

This year more than 90 percent of the freshman at George Mason Junior-Senior High, the only junior-senior high in the tiny Falls Church school district, are taking the year-long life science course, one of two major components of the sex education program. For juniors and seniors, the school offers a one-quarter course, called Seminar on Human Sexuality, and by the time they graduate, nearly half the seniors will have taken the seminar. Both courses are electives.

In both courses, students will discuss intercourse, contraception, abortion, pregnancy and homosexuality -- topics banned in other Northern schools. And, for the most part, those discussions will have the full-fledged support of parents.

"Falls Church is a small area where parents are personnally involved in their children's education, and we realize that sex is as much part of us as anything could be," said Ann Hoverson, who has two children in Falls Church schools.

Hoverson and her husband recently participated in a school-sponsored seminar for parents, designed to explain what the sex education program entails. More than 200 parents have enrolled in the course since its inception three years ago, according to school officials.

"My husband and I feel strongly that the more comfortable our children are in talking about sex, the more capable they will be of making a decision," Hoverson said. "If they understand the consequences of their actions, they will be far less likely to enter a relationship blindly. The better educated they are, the better able they wil be say no -- or yes -- when the time comes."

That time, according to national studies, may occur much sooner than most parents realize. Nearly eight out of 10 teen-aged boys and seven out of 10 teen-aged girls will have had intercourse by the time they are 19, according to a study recently released by the Guttmacher Institute, a private research group. Among 15- to 17-year-olds, nearly half the boys and one-third of the girls have had intercourse, according to the same study. Even more alarming for many parents is the institute's finding that four out of every 10 girls will become pregnant sometime during their teens.

In Northern Virginia, the most recent available figures show that more than 2,600 teen-aged girls reported pregnancies in 1978, including 140 girls younger than 15.

"Sex is no big deal. You either do it or you don't," said one Falls Church student, immediately following the sexuality seminar last week.

And he issued a warning to parents: "If you don't think your kids are having sex or at least thinking about it, you better think twice. Sex for teen-agers is here to stay."

In a private conversation, after the class, a 17-year-old student discussed her attitudes toward sex. The student, who described herself as "generally more conservative (than her classmates) and half 'goody-goody'" said she and her boyfriend frequently have oral sex, but never intercourse. "It's basically fear of pregnancy that has stopped us," she said.

Yet, despite statistical evidence that teen-agers are sexually active, Northern Virginia schools have been more than a little reluctant to approve sex education courses that would allow teen-agers to confront and discuss their own sexuality. Alexandria and Arlington do not have separate sex education programs, although Alexandria is considering one.

"One of the fiercest debates over sex education has occurred in Fairfax County. Three years ago, county schools considered expanding their sex education courses, now an afterschool program considered to be one of the most restrictive in the area. After more than 40 public hearings, school officials scrapped the plans. Next month, the issue will be debated once again. This time around, school officials took the precautionary step of surveying parents about their attitudes toward an expanded program. The results: parents overwhelmingly supported sex education courses, including discussion of the now-banned topic of birth control.

Mary Lee Tatum, who teaches the senior high seminar in Falls Church, is somewhat mystified by the opposition to sex education. "When you're dealing with people who are anti-sex education, you're talking about people who don't want to put sex anywhere. Not in the schools, not in the churches, not in the homes," says Tatum, a dynamic and frank woman who is regarded as a primary reason for the success of the Falls Church program. "These people tend to think that the minute you give permission to discuss sex in the classroom, you are automatically giving permission to be sexually active.

"That is certainly not the case. Studies have shown that less than 10 percent of high school students get any type of sex education, but that nearly half of all teen-agers are sexually active. How can you explain the other 40 percent?"

Then she asks: "Isn't sex education too important to leave to Jorache jeans?"

The Falls Church program is taught on three levels: sixth grade, nineth grade and the seminar for juniors and seniors. No questions are banned. The core of the program is taught on the freshman level where students complete a life science course, learning about all parts of the body and discussing every cycle of human life, from birth to death. There are no films, except a basic boy-grows-up-girl-grows-up in the sixth grade. The basic format for all the courses is a teacher-led seminar.

A key ingredient in the course is parental involvement, and a continued emphasis that the family is the most important link in sex education.

In a booklet provided to all Falls Church parents, the school district explains its goals: "Attitudes, values and behaviors are basically learned within the family setting. As a school system, it is our hope that we complement the home in its consideration of these essential issues [sexuality] of human life."

The booklet goes on to outline in detail what each course will include and, at every step, suggests topics for family discussion. For instance, in the fourth quarter guide for parents of nineth graders, the guide suggests that parents and children "discuss family attitudes and values toward controversial sex-related issues" or that they ask their "priest, pastor or rabbi to discuss moral stances the church or synagogue takes on sex-related issues."

Tatum regards the course as one of the most important ways that the family and schools, working together, can provide youngsters with a realistic view of human sexuality and the responsibilities she believes that inevitably entails.

"The kids are exposed to so much aberrant sexual behavior that sex education counteracts this media blitz we're seeing," Tatum says. "The course gives the kids an opportunity to talk about something more than the fantasy images they see constantly. Emotions are a very large part of the course."

Indeed, Tatum says the most frequent question she fields is: "How can I get him or her to like me?"

"They're curious about sex, but that is far from the major topic of interest," she notes.

During one recent morning seminar, the discussion rarely touched upon sex. Instead, the 25 high school students talked about how to better communicate with boyfriends or girlfriends and how to del with the ambivalent feelings of jealousy and attraction.

"I know my boyfriend likes it when I play the guitar, but at the same time he's jealous that I have talent," said one student. "I don't know what to do about it."

About 10 students responded to the young woman's statement. Some had similar problems, while others offered ways to cope. The young woman nodded, and indicated she would consider their advice.

Students agree with Tatum's assessment that the course teaches teen-agers how to make decisions, rather than forcing a decision too early.

"Before the class I wouldn't have been able to say no," said one young woman in the senior high class last week. "But now I could say, 'Hey, this is my body and if you want to have sex, fine, have it. But you can't have it with me'.

"I just hope that if I ever have a child that a class like this will be available."